The production of a spheroidally, or more particularly a spherically, contoured fabric of material which is difficult to weave, such as a carbon fiber material, has become desirable in recent times for use in the formation of shims for spherically shaped rocket nozzle parts, for use in the construction of parabolic antennae, and as bases or cores for radar domes made of resin or the like.
Ordinary yarn or thread, such as that made from cotton or wool or similar artificial yarns, can be formed into spheroidally contoured fabrics on conventional shuttle-type looms. However, these looms are not very well adapted for yarns or threads with different characteristics, such as low or medium modulus of elasticity carbon yarns, E-glass or S2-glass yarns, or other aerospace grade or egineered type yarns, which are more difficult to weave. These will hereinafter be referred to simply as "more difficult to weave" yarns. When these more difficult to weave yarns are run on a conventional shuttle loom, it is difficult to handle and does not produce sufficiently satisfactory fabric for the purpose of forming shims and the like. This is because when the warp yarns are formed into the shed and moved vertically past each other as the shed is changed during normal weaving to the degree this is done in the conventional shuttle-type loom, the yarns tend to fray or even break. Further, because the ordinary reciprocating shuttle carries a supply of weft or filler yarn thereon, it is difficult to satisfactorily operate the shuttle because the yarn is difficult to wind into the relatively small space on the shuttle, and it is bent extensively as it is unwound and passed through the shed of the warp yarns during the reciprocal motion of the shuttle. This also leads to fraying of the yarn and breakage to an extent which is not acceptable for normal production requirements.
A rapier-type loom, also variously called a needle type loom or a shuttleless loom, has been used for the weaving of contoured fabrics of another difficult to weave yarn, namely aramid, a thread or yarn sold under the trade name Kevlar by E.I. Dupont De Nemours & Co. Such a loom is also adaptable to weave the above-described difficult to weave yarns, because the rapier or needle is relatively small in cross section as compared with a conventional shuttle, moving through the shed to pick up a single length of filler or weft thread and then drawing it back through the shed, it is not necessary to move the warp yarns to the same extent during changing of the shed as in a conventional shuttle loom, thereby reducing the opportunity for fraying. Moreover, because the rapier simply picks up the end of a length of filler or weft yarn, and simply draws it across the shed during the return movement of the rapier, there is essentially no bending of the weft thread and thus little or no opportunity for breakage.
The rapier-type loom is therefore a desirable type loom for weaving of the highly heat insulating fiber material or fibers with similar characteristics.
However, the amount of contouring of the woven fabric which can be achieved by the usual rapier-type loom is relatively limited. The contouring is achieved by providing the desired contour on the peripheral surface of the mandrel which takes up the fabric after it has been woven at the shed area and which feeds the contoured fabric to a take-up roller on which the finished fabric is stored. The degree of contouring of the mandrel, however, must be limited. The reason for this is that at larger diameter portions of the mandrel, for example near the longitudinal middle of the mandrel, the warp yarns or threads coming from the shed area of the loom will be drawn in the direction of the mandrel along a shorter path and at a higher speed than warp yarns which extend around the smaller diameter portions of the mandrel, for example at the ends of the mandrel. This tends to cause the parts of the weft yarns or threads which are toward the edges of the fabric to lay behind the part in the center of the fabric so as to be changed from their normal direction perpendicular to the length of the warp yarns or threads to distorted positions in which they are at an angle to the warp yarns or threads. The greater the difference in diameter between the larger diameter portions and the smaller diameter portions of the mandrel, the more that the portions of the weft yarns or threads toward the edges will be caused to lag, and the greater will be the distortion of the position of the weft yarns or threads from the perpendicular position. When the distortion of the position of the weft yarns or threads becomes too great, the fabric becomes unsatisfactory.
This problem is accentuated when it is attempted to provide a profile for the mandrel which will produce a spheroidally shaped cloth, specifically a spherically shaped cloth, since the differences between the larger diameter portions and the smaller diameter portions become rather large. In the finished product, while the warp yarns or threads will be substantially circumferential around the axis of a sphere, in the nature of the latitude lines on a globe, the weft yarns or threads will not run in planes parallel to the axis of the sphere, in the nature of longitude lines on a globe, but rather will be caused to curve away from such positions from the larger diameter edge of the spherical portion to the smaller diameter edge. This problem will be discussed more fully hereinafter. This distortion of the position of the weft yarns or threads makes the fabric unsatisfactory.
Another property of spheroidally contoured fabric is that if it is shaped from fabric in which the weft yarns and warp yarns are uniformly spaced across the fabric, when the fabric is shaped into a spherical shape, the weft yarns will be closer together at the portion of the fabric nearer the pole of the sphere, i.e. the fibers lying along longitude lines of the sphere will converge toward the poles, so that the density of the fabric will increase toward the poles of the spheroid. This can be undesirable.